A modem is disclosed in copending application of O. Leon Pierce, Mark C. Smith, and R. Byron Driver, entitled Line Powered Modem filed contemporaneously herewith, and assigned to the same assignee as this invention, in which the entire operating power for the modem circuits is derived from the available telephone line power. This modem includes an RS232 output interface circuit which provides a carrier detect signal and a received data signal which must be within predetermined voltage ranges to be properly received by the associated business machine or other utilization apparatus to which the modem is connected. This output interface circuit must be appropriately powered to provide the intended output signal levels.